It still works pretty much like that, when we get to several gigahertz, obstructions start to affect more and more until you ultimately will have worse connections through moist air (that'll take double digit GHz's, though).
The 2.4 GHz area has problems, both because being nowadays so crowded with WLANs and all kinds of wireless products, and because it will be badly hit by microwave ovens.
But all this doesn't seem to bother DECT telephones that much, they seem to work much farther and better through obstructions than, say, WLAN. I believe this is because the data rate is much lower, which means that a much worse signal-to-noise ratio will still be enough to carry it. So there's not much need to move the telephones any higher - there's plenty of space for them at the current band, and the move might just create range problems in larger buildings with heavy walls.
EDIT: the cellular phones are a completely different thing - there, the lower frequencies really are getting too crowded, which is why they have moved from 450 MHz - 900 - 1800 - 1900 - 2100 and so on. The tradeoff is the cell size, it gets seriously small at the higher frequencies, which is why 3G is moving back down to 900 MHz while room is being made from other services. But these cordless home/office telephones really don't need mile-long ranges from the base 